Wafers formed of semiconductor material are needed for a variety of applications, and there is an ever-increasing demand for such wafers in most, if not all, of such applications.
Currently, silicon is the most commonly used semiconductor material for making semiconductor wafers. Accordingly, where the expression “semiconductor” or the expression “semiconductor material” is used herein, the discussion in particular relates to silicon. However, those of skill in the art will readily appreciate that in many instances, other semiconductor materials could be substituted for silicon with analogous results.
For example, solar-electric systems employ a semiconductor substrate, typically made of silicon (single crystal or polycrystalline), especially for deployment at or near the surface of the earth. Solar-electric systems have become more and more common, and of greater and greater importance. The use of solar-electric systems is expected to increase, potentially dramatically. As such, improvements in solar-electric technology, even incremental improvements, are of great importance. Although the expression “solar-electric” is used herein, persons of skill in the art will recognize that the discussion applies to all kinds of photovoltaic materials, systems and phenomena.
A significant portion of the cost of such semiconductor wafers is the raw semiconductor material itself. For example, in the case of solar-electric systems, a primary factor which has limited the use of such systems is the cost of the semiconductor material in the semiconductor wafers (in particular, silicon) needed for such systems.
A wide variety of methods exist for producing semiconductor wafers. For example, polycrystalline silicon wafers for use in solar cells have been produced by melting a high-purity material to which a dopant such as phosphorus, boron, gallium, antimony or the like is added, has been added, or will be added, in an inert atmosphere in a crucible, depositing the resulting silicon melt and cooling to form a polycrystalline ingot, and slicing the ingot with a wire saw or an inner diameter blade, thereby obtaining wafers. In such processes, a great deal of silicon is lost as kerf during such slicing, bringing about a potentially significant reduction in yield. In addition, sawing is capable only of providing a substantially straight cut, i.e., any patterning on the cut surfaces (or internal structuring within the wafer) has to be accomplished in a subsequent step, e.g., by grinding, etching, lasering, drilling, etc. Such grinding, etching, lasering or drilling results in further losses of semiconductor material, requires time for such processing, and requires expensive equipment.
In another group of processes, e.g., as disclosed in U.S. Pat. No. RE 36,156, granular silicon is applied to a belt or a setter, the silicon (along with the belt or setter) is then subjected to a thermal sequence to form a sheet of silicon, the sheet is then removed from the belt or setter, and the sheet is then sized by sawing or scribing. Such processes likewise involve losses due to sawing or scribing, such sawing and scribing requires significant effort to obtain accuracy, and any patterning on the surfaces of the wafers or internal structuring of the wafers generally has to be accomplished in a subsequent step, e.g., by grinding, etching, lasering, drilling, etc.
There is an ongoing need for a continuous method, and apparatus for use in carrying out such a method, for converting semiconductor material, e.g., silicon, into wafers, which method minimizes consumption of the semiconductor material, which produces high quality wafers and which can be employed using lower grade raw semiconductor material.
In particular, there is a need for a method and apparatus as described above which can produce wafers having low residual stress. Such wafer must have at least acceptable electrical and optical properties. There is further a need for such wafers which have improved or outstanding electrical and optical properties.
It would further be highly desirable to be able to produce wafers having any desired three-dimensional shape, without having to carry out post-fabrication steps (e.g., sawing, grinding, etching, lasering, drilling, etc.) to form such a shape.
In addition, it would be desirable to provide a method, or apparatus for use in carrying out such a method, in which overall wafer uniformity is improved, and in which thermal treatment of the semiconductor material out of which the wafers are manufactured can be more closely controlled.